Star Formation in Sculptor Group Dwarf Irregular Galaxies and the Nature of “Transition” Galaxies

We present new Hα narrow band imaging of the HII regions in eight Sculptor Group dwarf irregular (dI) galaxies. The Hα luminosities of the detected HII regions range from some of the faintest detected in extragalactic HII regions (∼ 10 erg s in SC 24) to some of the most luminous (∼ 10 erg s in NGC 625). The total Hα luminosities are converted into current star formation rates (SFR). Comparing the Sculptor Group dIs to the Local Group dIs, we find that the Sculptor Group dIs have, on average, lower values of SFR when normalized to either galaxy luminosity or gas mass (although there is considerable overlap between the two samples). The range for both the Sculptor Group and Local Group samples is large when compared to that seen for the sample of gas-rich quiescent low surface brightness (LSB) dIs from van Zee et al. (1997) and the sample of isolated dIs from van Zee (2000, 2001). This is probably best understood as a selection effect since the nearby group samples have a much larger fraction of extremely low luminosity galaxies and the smaller galaxies are much more liable to large relative variations in current SFR. The Sculptor Group and LSB samples are very similar with regard to mean values of both τgas and τform, and the Local Group and isolated dI samples are also similar to each other in these two quantities. Presently, the Sculptor Group lacks dI galaxies with elevated normalized current SFRs as high as the Local Group dIs IC 10 and GR 8.